EP0081114A2 - Method of firing a pulverized fuel-fired steam generator - Google Patents
Method of firing a pulverized fuel-fired steam generator Download PDFInfo
- Publication number
- EP0081114A2 EP0081114A2 EP82110650A EP82110650A EP0081114A2 EP 0081114 A2 EP0081114 A2 EP 0081114A2 EP 82110650 A EP82110650 A EP 82110650A EP 82110650 A EP82110650 A EP 82110650A EP 0081114 A2 EP0081114 A2 EP 0081114A2
- Authority
- EP
- European Patent Office
- Prior art keywords
- mill
- flue gas
- gaseous mixture
- fuel
- air
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Granted
Links
- 238000000034 method Methods 0.000 title claims abstract description 11
- 238000010304 firing Methods 0.000 title claims abstract description 9
- 239000000446 fuel Substances 0.000 claims abstract description 57
- 239000003546 flue gas Substances 0.000 claims abstract description 51
- UGFAIRIUMAVXCW-UHFFFAOYSA-N Carbon monoxide Chemical compound [O+]#[C-] UGFAIRIUMAVXCW-UHFFFAOYSA-N 0.000 claims abstract description 48
- 239000008246 gaseous mixture Substances 0.000 claims abstract description 47
- 239000001301 oxygen Substances 0.000 claims abstract description 26
- 229910052760 oxygen Inorganic materials 0.000 claims abstract description 26
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 claims abstract description 25
- 239000003570 air Substances 0.000 claims description 62
- 238000010438 heat treatment Methods 0.000 claims description 5
- 239000002245 particle Substances 0.000 claims description 5
- 239000004449 solid propellant Substances 0.000 claims description 4
- 238000010298 pulverizing process Methods 0.000 claims description 3
- 239000012080 ambient air Substances 0.000 claims description 2
- 238000005259 measurement Methods 0.000 claims description 2
- 238000002156 mixing Methods 0.000 claims description 2
- 238000001816 cooling Methods 0.000 claims 2
- 230000003134 recirculating effect Effects 0.000 claims 1
- 239000000203 mixture Substances 0.000 abstract description 19
- 238000001035 drying Methods 0.000 abstract description 10
- MWUXSHHQAYIFBG-UHFFFAOYSA-N Nitric oxide Chemical compound O=[N] MWUXSHHQAYIFBG-UHFFFAOYSA-N 0.000 description 21
- 239000003245 coal Substances 0.000 description 19
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 15
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 13
- 239000007789 gas Substances 0.000 description 12
- 238000002485 combustion reaction Methods 0.000 description 10
- 229910052757 nitrogen Inorganic materials 0.000 description 8
- 230000015572 biosynthetic process Effects 0.000 description 6
- 239000002609 medium Substances 0.000 description 6
- 230000007423 decrease Effects 0.000 description 5
- 239000002737 fuel gas Substances 0.000 description 3
- 239000006163 transport media Substances 0.000 description 3
- 239000000470 constituent Substances 0.000 description 2
- 230000001419 dependent effect Effects 0.000 description 2
- 238000006243 chemical reaction Methods 0.000 description 1
- 239000003344 environmental pollutant Substances 0.000 description 1
- 239000002803 fossil fuel Substances 0.000 description 1
- 238000004952 furnace firing Methods 0.000 description 1
- 239000003077 lignite Substances 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 150000002829 nitrogen Chemical class 0.000 description 1
- 230000003647 oxidation Effects 0.000 description 1
- 238000007254 oxidation reaction Methods 0.000 description 1
- 150000002926 oxygen Chemical class 0.000 description 1
- 239000013618 particulate matter Substances 0.000 description 1
- 239000003415 peat Substances 0.000 description 1
- 231100000719 pollutant Toxicity 0.000 description 1
- 239000000843 powder Substances 0.000 description 1
- 239000007787 solid Substances 0.000 description 1
Images
Classifications
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
- F23N—REGULATING OR CONTROLLING COMBUSTION
- F23N1/00—Regulating fuel supply
- F23N1/02—Regulating fuel supply conjointly with air supply
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
- F23C—METHODS OR APPARATUS FOR COMBUSTION USING FLUID FUEL OR SOLID FUEL SUSPENDED IN A CARRIER GAS OR AIR
- F23C9/00—Combustion apparatus characterised by arrangements for returning combustion products or flue gases to the combustion chamber
- F23C9/003—Combustion apparatus characterised by arrangements for returning combustion products or flue gases to the combustion chamber for pulverulent fuel
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
- F23K—FEEDING FUEL TO COMBUSTION APPARATUS
- F23K1/00—Preparation of lump or pulverulent fuel in readiness for delivery to combustion apparatus
- F23K1/04—Heating fuel prior to delivery to combustion apparatus
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
- F23K—FEEDING FUEL TO COMBUSTION APPARATUS
- F23K3/00—Feeding or distributing of lump or pulverulent fuel to combustion apparatus
- F23K3/02—Pneumatic feeding arrangements, i.e. by air blast
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
- F23L—SUPPLYING AIR OR NON-COMBUSTIBLE LIQUIDS OR GASES TO COMBUSTION APPARATUS IN GENERAL ; VALVES OR DAMPERS SPECIALLY ADAPTED FOR CONTROLLING AIR SUPPLY OR DRAUGHT IN COMBUSTION APPARATUS; INDUCING DRAUGHT IN COMBUSTION APPARATUS; TOPS FOR CHIMNEYS OR VENTILATING SHAFTS; TERMINALS FOR FLUES
- F23L15/00—Heating of air supplied for combustion
- F23L15/04—Arrangements of recuperators
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
- F23N—REGULATING OR CONTROLLING COMBUSTION
- F23N2221/00—Pretreatment or prehandling
- F23N2221/12—Recycling exhaust gases
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E20/00—Combustion technologies with mitigation potential
- Y02E20/34—Indirect CO2mitigation, i.e. by acting on non CO2directly related matters of the process, e.g. pre-heating or heat recovery
Definitions
- the present invention relates generally to the operation of pulverized fuel-fired steam generator furnaces and, more particularly, to a method of firing a pulverized fuel-fired steam generator furnace by conveying the fuel pulverized in the mill to the furnace entrained in a gaseous mixture of recirculated flue gas and air wherein the volume flow rate of the gaseous mixture is controlled in response to the feed rate of fuel to the mill and the volume flow rate of air in the gaseous mixture is controlled to maintain a desired oxygen level in the gaseous mixture entering the mill.
- the fuel In firing solid fossil fuels, such as coal, lignite and peat, the fuel must be comminuted and dried before it can be introduced into the furnace. This is accomplished in the mill wherein the solid fuel is simultaneously pulverized and substantially dried. In order to obtain sufficient heat for adequate drying, a hot gaseous medium is admitted to the mill in a quantity necessary to provide sufficient heat to evaporate moisture in the fuel. The same gaseous medium is then used to transport the pulverized coal from the mill to the furnace for combustion therein.
- a hot gaseous medium is admitted to the mill in a quantity necessary to provide sufficient heat to evaporate moisture in the fuel. The same gaseous medium is then used to transport the pulverized coal from the mill to the furnace for combustion therein.
- the gaseous medium supplied to tne mill for drying and subsequent transport of the pulverized fuel to the furnace is preheated air.
- this oxygen is readily available to oxidize constituents in the pulverized fuel.
- One constituent of pulverized fuel is nitrogen bound in the complex organic structure of the fuel. This nitrogen tends to readily combine with oxygen in the earlier stages of combustion to form nitric oxide, a major pollutant.
- a high air to coal ratio in the transport stream could lead to increased formation of nitrogen oxide from fuel bound nitrogen.
- the volume flow rate of preheated air to the mill is controlled in response to the feed rate of fuel to the mill.
- the ratio of the flow rate of air to the feed rate of fuel changes over load with the ratio of air flow rate to the fuel feed rate increasing as load on the steam generator, and therefore fuel feed rate, decreases.
- the availablity of oxygen in the pulverized fuel-air stream being conveyed to the furnace increases. That is, the air to fuel ratio, and therefore the pounds of oxygen available per pound of fuel, increases. This increase in air to fuel ratio as load decreases leads to a further increase in formation of nitrogen oxide from fuel bound nitrogen at low load.
- a furthur object of the present invention is to limit nitric oxide formation from fuel bound nitrogen via control ting oxygen availablity in the transport medium.
- Still another object of the present invention is to maintain efficient drying within the mill while using a recirculated fuel gas and air mixture as the drying medium.
- a method of firing a pulverized fuel-fired steam generator furnace comprising feeding solid fuel to the mill for pulverizing therein at a fuel feed rate controlled in response to load demand on the steam generator, supplying a mixture of recirculated cleaned flue gas and air to the mill for drying the pulverized fuel in the mill and conveying the pulverized fuel to the furnace, controlling the volume flow rate of the gaseous mixture of recirculated flue gas and air in response to the feed rate of fuel to the mill, to maintain a gaseous mixture to fuel weight ratio in the gaseous mixture of about but not less than 1.5.
- the oxygen content of the gaseous mixture of recirculated flue gas and air entering the mill I is measured and the. volume flow rate of recirculated flue gas and the volume flow rate of air controlled with respect to each other in response to the oxygen measurement so as to maintain the oxygen level in the gaseous mixture entering the mill at a level of at least 12% by volume and preferably in the range of 12 to 15% by volume.
- Figure 1 is a sectional side elevational view showing a pulverized fuel-fired steam generator fired in accordance with the present invention
- Figure 2 is an enlarged side elevation view showing the means for controlling fuel, air, and recirculated gas flow in accordance with the present invention.
- a pulverized fuel-fired steam generator having a furnace 10 formed of water walls 12.
- water is passed upwardly through the water walls 12 wherein it absorbs heat from the combustion of fuel within the furnace 10.
- the water is first heated to saturation temperature and then partially evaporated to form a steam-water mixture.
- the steam-water mixture leaving the water walls 12 is collected in an-outtet header and passed to drum 14 wherein the steam and water are separated.
- the water separated from the steam-water mixture in the drum 14 is mixed with feed water and recirculated through the water walls 12.
- the steam removed from the steam-water mixture in the drum 14 is passed through heat exchange surface 16, such as superheat and reheat surface, disposed in the gas exit duct 18 which interconnects the furnace 10 with the steam generator stack for providing a flow passage for conveying the gases formed in the furnace to the stack.
- heat exchange surface 16 In passing through the heat exchange surface 16, the steam is heated as it passes in heat exchange relationship with the hot flue gases generated in the furnace 10 and leaving the furnace 10 through exit duct 18.
- the hot flue gas leaving the furnace 10 through gas exit duct 18 traverses the steam heating surface 16 disposed therein, the hot flue gas is cooled by transtering heat to the steam flowing through the steam heating surface 16 to a temperature typically in the range of 320 to 370 C.
- the flue gas is then typically further cooled to a temperature in the range of 120 to 150 C by passing the flue gas in heat exchange relationship with combustion air being supplied to the furnace 10 through air preheater 20 disposed downstream of the furnace 10 in the gas exit duct 18.
- Also disposed downstream of the furnace 10 in gas exit duct 18 is a particulate collector 22 wherein ash particles and other particulate matter entrained in the flue gas during the combustion process is removed therefrom.
- the cool clean flue gas leaving the particulate collector 22 is passed through fan 24 and vented to the atmosphere . via stack 26.
- the fan 24 boosts the pressure of the cool clean flue gas before it is vented to the atmosphere.
- the furnace 10 is fired by injecting pulverized fuel Into the furnace by burners 28 disposed in windboxes 30.
- Combustion air which has been preheated by passing in heat exchange relationship in air preheater 20 with the flue gases leaving the furnace 10 through duct 18, is supplied through duct 32 to the wind box 30 for introduction to the furnace 10.
- the amount of fuel injected into the furnace is controlled in response to load demand on the steam generator to provide the total heat release necessary to yield a desired steam generation for the given steam generator design.
- solid fuel such as raw coal
- pulverizer 36 wherein the fuel is comminuted to a fine powder like particle size.
- preheated air is supplied to the pulverizer 36 from the air preheater outlet. As the preheated air sweeps through the pulverizer 36, the comminated coal is entrained therein and dried by the preheated air stream. The preheated air used in drying the pulverized coal is also used to transport the pulverized coal to the burners 28.
- the air used to dry the pulverized coal and transport the coal to the burners is typically 10 to 15% of the total combustion air supplied to the furnace 10 through windbox 30.
- the preheated air used in dryinq the pulverized coal and transfering the coal to the furnace increases to 20% to 30% of the total combustion air.
- the amount of fuel supplied to the furnace 10 as load decreases also decreases. Therefore, on a typical coal fired furnace, the ratio on a weight basis of the air flow rate to pulverizers 36 to the fuel feed rate thereto increases from about 1.5 at full load to as high as 3 or 4 at low load.
- this increased oxygen availability in the transport stream at low loads it is thought that a greater amount of nitrogen inherently bound in the coal is converted to nitrooen oxides at low load than is converted at high loads.
- the present invention In order to reduce the conversion of fuel bound nitrogen to nitrogen oxide at low load, it is contemplated by the present invention to control the amount of air, and therefore the oxygen content, in the transport stream by substituting cooled clean recirculated flue gas for a portion of the air flow normally supplied to the pulverizer 36.
- a portion of the cooled clean flue gas passing from the outlet of booster fan 24 to the stack 26 is recirculated through duct 38 and mixed with the ambient air supply for the pulverizer 36.
- the air and recirculated flue gas mixture is then conveyed through duct 40 to the pulverizer 36 by fan 42.
- air and recirculated flue gas mixture traverses duct 40 it passes through air preheater 20 where it is passed in heat exchange relationship with the flue gases leaving the furnace 10 through exit duct 18 and is therefore preheated typically to a temperature in the range of 260 to 400 C.
- a portion of the air and fuel gas mixture may be bypassed around the air heater 20 through duct 44 and is remixed with the preheated air and flue gas mixture as a means of fine tuning the temperature of the gaseous mixture supplied to the mill 36.
- the control of the furnace firing process may be obtained through a series of dampers and controllers as best illustrated in Figure 2.
- a master signal 3 indicative of steam generator load is sent to fuel-feed controller 50.
- the fuel feed controller 50 generates and transmits a signal 51 to feeder 34 which in response thereto regulates the rate of fuel feed to the pulverizer 36.
- the controller 50 generates and transmits a second signal 53 indicative of the fuel feed rate to mixture volume flow rate controller 70.
- the air and fuel gas flow controller 60 receives a signal 55 from the oxygen monitor 52 disposed in gas duct 40 at the inlet to the mill 36.
- Signal 55 is indicative of the oxygen content of the air and recirculated flue gas mixture entering the mill 36.
- the controller 60 sends a control signal 61 to damoer drive 62 to selectively open or close flue gas damper 64 disposed in flue gas recirculation duct 38.
- the flue gas damper 64 is manipulated by controller 60 so as to maintain the oxygen level in the gaseous mixture entering the mill 36 at a level of at least 12% by volume and preferably within the range of 12 to 15% by volume. It is important that the oxygen level in the gaseous mixture entering the mill stay above 12% as combustion instability will occur within the furnace if the oxygen level in the transport stream drops below 12%. Further, it is advisable to keep the oxygen level in the transport stream in the range of 12 to 15% inorder to reduce nitric oxide formation from the oxidation of fuel bound nitrogen. If air only were used as a transport medium, the oxygen level would be approximately 21% by volume.
- a volume flow monitor 56 is disposed in the gas duct 40 near the inlet to the mill 36 to monitor the volume flow rate of the gaseous mixture of air and recirculated flue gas therethrough.
- Flow monitor 56 sends a signal 59 indicative of the volume flow rate of gaseous mixture to the mill to controller 70.
- the controller 70 sends a control signal 71 to damper drive 72 and a control signal 73 to damper drive 74 to selectively open or close dampers 76 and 78 respectively.
- Controller 70 is pre- programed to maintain the volume flow rate of the gaseous mixture of air and recirculated flue gas to the mill at a preselected value dependent upon the instantaneous fuel feed rate. On a weight basis, the ratio of the flow of gaseous mixture to fuel should be maintained at a value greater than about 1.5 in order to ensure that there is sufficient volume of gaseous mixture to dry the coal and transport the coal to the furnace.
- the gaseous mixture supplied to the mill 36 must be at a sufficient temperature to provide enough heat to evaporate moisture contained in the fuel pulverized in the mill 36. Additionally, the temperature of the gaseous mixture leaving the mill must be high enough to ensure that the moisture evaporated from the fuel does not condense as the fuel is being conveyed to the furnace. Therefore, a temperature monitor 90 is disposed at the outlet of the mill 36 to monitor the temperature of the gaseous mixture transporting the pulverized fuel from the mill to the burners 28.
- the temperature monitor 90 generates and transmits a signal 93 indicative of the mill outlet gas temperature to the gaseous mixture temperature controller 80.
- the controller 80 generates and transmits a control signal 81 to damper drive 72 and a control signal 83 to damper drive 74.
- Damper drive 72 actuates damper 76 disposed in gas duct 40 at a location just downstream of the air preheater 20.
- Damper drive 74 actuates damper 78 disposed in gas duct 44 which is the bypass duct for bypassing a portion of the gaseous mixture around the air preheater.
- Controllers 70 and 80 cooperate with each other by means of feed back signals so that dampers 76 and 78 may be selectively opened or closed to maintain both the volume flow rate of the gaseous mixture to the mill at a preselected level dependent upon fuel feed rate and also simultaneously to maintain temperature of the gaseous mixture entering the mill at a level sufficient to ensure that the temperature mixture leaving the mill is at about 85 C.
- damper 76 and 78 By selectively positioning damper 76 and 78, the gaseous mixture of recirculated flue gas and air is separated into a first portion which is passed in heat exchange relationship with the flue gas leaving the furnace through the air preheater 20 and a second portion which is not preheated but bypasses the air preheater 20 through duct 44 to remix with the preheated gaseous mixture passing through duct 40 at a location downstream of damper 76 prior to entering the mill 36.
- the damper 78 By selectively positioning the damper 78, the temperature of the gases of the preheated gaseous mixture leaving air heater 20 may be tempered with an unheated gaseous mixture to control mill outlet temperature.
- the applicant has provided an improved method of firing a pulverized fuel-fired furnace wherein the oxygen content of the gaseous mixture supplied to the mill for drying the pulverized coal and transporting the pulverized coal to the furnace can be controlled to yield reduced nitrogen oxide formation by mixing recirculated flue gas with air through an intergrated control system while simultaneously ensuring ignition stability by maintaining a safe level of oxygen and air in the gaseous mixture.
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Abstract
Description
- The present invention relates generally to the operation of pulverized fuel-fired steam generator furnaces and, more particularly, to a method of firing a pulverized fuel-fired steam generator furnace by conveying the fuel pulverized in the mill to the furnace entrained in a gaseous mixture of recirculated flue gas and air wherein the volume flow rate of the gaseous mixture is controlled in response to the feed rate of fuel to the mill and the volume flow rate of air in the gaseous mixture is controlled to maintain a desired oxygen level in the gaseous mixture entering the mill.
- In firing solid fossil fuels, such as coal, lignite and peat, the fuel must be comminuted and dried before it can be introduced into the furnace. This is accomplished in the mill wherein the solid fuel is simultaneously pulverized and substantially dried. In order to obtain sufficient heat for adequate drying, a hot gaseous medium is admitted to the mill in a quantity necessary to provide sufficient heat to evaporate moisture in the fuel. The same gaseous medium is then used to transport the pulverized coal from the mill to the furnace for combustion therein.
- Typically, the gaseous medium supplied to tne mill for drying and subsequent transport of the pulverized fuel to the furnace is preheated air. As the pulverized fuel-air mixture enters the furnace and combustion begins, this oxygen is readily available to oxidize constituents in the pulverized fuel. One constituent of pulverized fuel is nitrogen bound in the complex organic structure of the fuel. This nitrogen tends to readily combine with oxygen in the earlier stages of combustion to form nitric oxide, a major pollutant. Thus there is concern that a high air to coal ratio in the transport stream could lead to increased formation of nitrogen oxide from fuel bound nitrogen.
- Typically, the volume flow rate of preheated air to the mill is controlled in response to the feed rate of fuel to the mill. Also, the ratio of the flow rate of air to the feed rate of fuel changes over load with the ratio of air flow rate to the fuel feed rate increasing as load on the steam generator, and therefore fuel feed rate, decreases. As a consequence of the increase in the air flow rate to fuel feed rate as load decreases, the availablity of oxygen in the pulverized fuel-air stream being conveyed to the furnace increases. That is, the air to fuel ratio, and therefore the pounds of oxygen available per pound of fuel, increases. This increase in air to fuel ratio as load decreases leads to a further increase in formation of nitrogen oxide from fuel bound nitrogen at low load.
- One proposed solution to the problem of increased formation of nitrogen oxide at high air to fuel ratios in the transport stream has been to substitute recirculated flue gas for air as the gaseous medium feed to the mill to dry and transport the pulverized fuel. However, experience has shown that substituting recirculated flue gas for air as the transport medium led to ignition instability problems upon admission of the pulverized fuel-recirculated flue gas mixture to the furnace.
- It is therefore an object of the present invention to provide a method of firing a pulverized fuel-steam generator wherein a mixture of recirculated flue gas and air can be used as the drying and conveying medium in the mill while ensuring satisfactory ignition stability.
- A furthur object of the present invention is to limit nitric oxide formation from fuel bound nitrogen via control ting oxygen availablity in the transport medium.
- Still another object of the present invention is to maintain efficient drying within the mill while using a recirculated fuel gas and air mixture as the drying medium.
- With the afore mentioned objects in view, there is provided in accordance with the present invention a method of firing a pulverized fuel-fired steam generator furnace comprising feeding solid fuel to the mill for pulverizing therein at a fuel feed rate controlled in response to load demand on the steam generator, supplying a mixture of recirculated cleaned flue gas and air to the mill for drying the pulverized fuel in the mill and conveying the pulverized fuel to the furnace, controlling the volume flow rate of the gaseous mixture of recirculated flue gas and air in response to the feed rate of fuel to the mill, to maintain a gaseous mixture to fuel weight ratio in the gaseous mixture of about but not less than 1.5.
- The oxygen content of the gaseous mixture of recirculated flue gas and air entering the mill I is measured and the. volume flow rate of recirculated flue gas and the volume flow rate of air controlled with respect to each other in response to the oxygen measurement so as to maintain the oxygen level in the gaseous mixture entering the mill at a level of at least 12% by volume and preferably in the range of 12 to 15% by volume.
- Figure 1 is a sectional side elevational view showing a pulverized fuel-fired steam generator fired in accordance with the present invention, and
- Figure 2 is an enlarged side elevation view showing the means for controlling fuel, air, and recirculated gas flow in accordance with the present invention.
- Referring now to the drawing and particularity to Figure 1, there is depicted therein a pulverized fuel-fired steam generator having a
furnace 10 formed ofwater walls 12. To generate steam, water is passed upwardly through thewater walls 12 wherein it absorbs heat from the combustion of fuel within thefurnace 10. The water is first heated to saturation temperature and then partially evaporated to form a steam-water mixture. The steam-water mixture leaving thewater walls 12 is collected in an-outtet header and passed to drum 14 wherein the steam and water are separated. - The water separated from the steam-water mixture in the drum 14 is mixed with feed water and recirculated through the
water walls 12. The steam removed from the steam-water mixture in the drum 14 is passed throughheat exchange surface 16, such as superheat and reheat surface, disposed in thegas exit duct 18 which interconnects thefurnace 10 with the steam generator stack for providing a flow passage for conveying the gases formed in the furnace to the stack. In passing through theheat exchange surface 16, the steam is heated as it passes in heat exchange relationship with the hot flue gases generated in thefurnace 10 and leaving thefurnace 10 throughexit duct 18. - As the hot flue gas leaving the
furnace 10 throughgas exit duct 18 traverses thesteam heating surface 16 disposed therein, the hot flue gas is cooled by transtering heat to the steam flowing through thesteam heating surface 16 to a temperature typically in the range of 320 to 370 C. The flue gas is then typically further cooled to a temperature in the range of 120 to 150 C by passing the flue gas in heat exchange relationship with combustion air being supplied to thefurnace 10 throughair preheater 20 disposed downstream of thefurnace 10 in thegas exit duct 18. Also disposed downstream of thefurnace 10 ingas exit duct 18 is aparticulate collector 22 wherein ash particles and other particulate matter entrained in the flue gas during the combustion process is removed therefrom. The cool clean flue gas leaving theparticulate collector 22 is passed throughfan 24 and vented to the atmosphere.viastack 26. Thefan 24 boosts the pressure of the cool clean flue gas before it is vented to the atmosphere. - The
furnace 10 is fired by injecting pulverized fuel Into the furnace byburners 28 disposed inwindboxes 30. Combustion air, which has been preheated by passing in heat exchange relationship inair preheater 20 with the flue gases leaving thefurnace 10 throughduct 18, is supplied throughduct 32 to thewind box 30 for introduction to thefurnace 10. In accordance with conventional practice, the amount of fuel injected into the furnace is controlled in response to load demand on the steam generator to provide the total heat release necessary to yield a desired steam generation for the given steam generator design. - In pulverized fuel firing, as shown in the drawing, solid fuel, such as raw coal, is fed from a storage bin (not shown) at a controlled rate through
feeder 34 topulverizer 36 wherein the fuel is comminuted to a fine powder like particle size. In a typical pulverized fuel fired furnace, preheated air is supplied to thepulverizer 36 from the air preheater outlet. As the preheated air sweeps through thepulverizer 36, the comminated coal is entrained therein and dried by the preheated air stream. The preheated air used in drying the pulverized coal is also used to transport the pulverized coal to theburners 28. - At full load, the air used to dry the pulverized coal and transport the coal to the burners is typically 10 to 15% of the total combustion air supplied to the
furnace 10 throughwindbox 30. However, at low loads the preheated air used in dryinq the pulverized coal and transfering the coal to the furnace increases to 20% to 30% of the total combustion air. Naturally, the amount of fuel supplied to thefurnace 10 as load decreases also decreases. Therefore, on a typical coal fired furnace, the ratio on a weight basis of the air flow rate topulverizers 36 to the fuel feed rate thereto increases from about 1.5 at full load to as high as 3 or 4 at low load. As a result of this increased oxygen availability in the transport stream at low loads, it is thought that a greater amount of nitrogen inherently bound in the coal is converted to nitrooen oxides at low load than is converted at high loads. - In order to reduce the conversion of fuel bound nitrogen to nitrogen oxide at low load, it is contemplated by the present invention to control the amount of air, and therefore the oxygen content, in the transport stream by substituting cooled clean recirculated flue gas for a portion of the air flow normally supplied to the
pulverizer 36. In accordance with the present invention, a portion of the cooled clean flue gas passing from the outlet ofbooster fan 24 to thestack 26 is recirculated throughduct 38 and mixed with the ambient air supply for thepulverizer 36. The air and recirculated flue gas mixture is then conveyed throughduct 40 to thepulverizer 36 byfan 42. - As the air and recirculated flue gas mixture traverses
duct 40 it passes throughair preheater 20 where it is passed in heat exchange relationship with the flue gases leaving thefurnace 10 throughexit duct 18 and is therefore preheated typically to a temperature in the range of 260 to 400 C. As will be described in more detail later, a portion of the air and fuel gas mixture may be bypassed around theair heater 20 throughduct 44 and is remixed with the preheated air and flue gas mixture as a means of fine tuning the temperature of the gaseous mixture supplied to themill 36. - The control of the furnace firing process may be obtained through a series of dampers and controllers as best illustrated in Figure 2. A master signal 3 indicative of steam generator load is sent to fuel-
feed controller 50. In response thereto thefuel feed controller 50 generates and transmits asignal 51 tofeeder 34 which in response thereto regulates the rate of fuel feed to thepulverizer 36. Additionally, thecontroller 50 generates and transmits asecond signal 53 indicative of the fuel feed rate to mixture volumeflow rate controller 70. - The air and fuel gas flow controller 60 receives a
signal 55 from theoxygen monitor 52 disposed ingas duct 40 at the inlet to themill 36.Signal 55 is indicative of the oxygen content of the air and recirculated flue gas mixture entering themill 36. In response tosignal 55, the controller 60 sends acontrol signal 61 todamoer drive 62 to selectively open or closeflue gas damper 64 disposed in fluegas recirculation duct 38. - In accordance with the present invention, the
flue gas damper 64 is manipulated by controller 60 so as to maintain the oxygen level in the gaseous mixture entering themill 36 at a level of at least 12% by volume and preferably within the range of 12 to 15% by volume. It is important that the oxygen level in the gaseous mixture entering the mill stay above 12% as combustion instability will occur within the furnace if the oxygen level in the transport stream drops below 12%. Further, it is advisable to keep the oxygen level in the transport stream in the range of 12 to 15% inorder to reduce nitric oxide formation from the oxidation of fuel bound nitrogen. If air only were used as a transport medium, the oxygen level would be approximately 21% by volume. - A volume flow monitor 56 is disposed in the
gas duct 40 near the inlet to themill 36 to monitor the volume flow rate of the gaseous mixture of air and recirculated flue gas therethrough. Flow monitor 56 sends asignal 59 indicative of the volume flow rate of gaseous mixture to the mill tocontroller 70. In response to signal 53 indicative of fuel feed rate and signal 59 indicative of mixture volume flow rate, thecontroller 70 sends acontrol signal 71 to damper drive 72 and a control signal 73 to damper drive 74 to selectively open orclose dampers Controller 70 is pre- programed to maintain the volume flow rate of the gaseous mixture of air and recirculated flue gas to the mill at a preselected value dependent upon the instantaneous fuel feed rate. On a weight basis, the ratio of the flow of gaseous mixture to fuel should be maintained at a value greater than about 1.5 in order to ensure that there is sufficient volume of gaseous mixture to dry the coal and transport the coal to the furnace. - In order to ensure proper drying of the pulverized coal in the
mill 36, the gaseous mixture supplied to themill 36 must be at a sufficient temperature to provide enough heat to evaporate moisture contained in the fuel pulverized in themill 36. Additionally, the temperature of the gaseous mixture leaving the mill must be high enough to ensure that the moisture evaporated from the fuel does not condense as the fuel is being conveyed to the furnace. Therefore, a temperature monitor 90 is disposed at the outlet of themill 36 to monitor the temperature of the gaseous mixture transporting the pulverized fuel from the mill to theburners 28. - The temperature monitor 90 generates and transmits a signal 93 indicative of the mill outlet gas temperature to the gaseous mixture temperature controller 80. In response thereto, the controller 80 generates and transmits a control signal 81 to damper drive 72 and a
control signal 83 to damper drive 74.Damper drive 72 actuatesdamper 76 disposed ingas duct 40 at a location just downstream of theair preheater 20. Damper drive 74 actuatesdamper 78 disposed ingas duct 44 which is the bypass duct for bypassing a portion of the gaseous mixture around the air preheater. By selectively opening andclosing dampers mill 36 at a level of about 85 C. -
Controllers 70 and 80 cooperate with each other by means of feed back signals so thatdampers damper air preheater 20 and a second portion which is not preheated but bypasses theair preheater 20 throughduct 44 to remix with the preheated gaseous mixture passing throughduct 40 at a location downstream ofdamper 76 prior to entering themill 36. By selectively positioning thedamper 78, the temperature of the gases of the preheated gaseous mixture leavingair heater 20 may be tempered with an unheated gaseous mixture to control mill outlet temperature. - Accordingly, it will be appreciated that the applicant has provided an improved method of firing a pulverized fuel-fired furnace wherein the oxygen content of the gaseous mixture supplied to the mill for drying the pulverized coal and transporting the pulverized coal to the furnace can be controlled to yield reduced nitrogen oxide formation by mixing recirculated flue gas with air through an intergrated control system while simultaneously ensuring ignition stability by maintaining a safe level of oxygen and air in the gaseous mixture.
- While the applicant has illustrated and described herein the preferred embodient of his invention, it is to be understood that such is merely illustrative and not restrictive in that variations and modifications by those skilled in the art may be made thereto without departing from the scope and spirit of the invention as recited in the claims apended hereto.
Claims (4)
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US327844 | 1981-12-07 | ||
US06/327,844 US4411204A (en) | 1981-12-07 | 1981-12-07 | Method of firing a pulverized fuel-fired steam generator |
Publications (3)
Publication Number | Publication Date |
---|---|
EP0081114A2 true EP0081114A2 (en) | 1983-06-15 |
EP0081114A3 EP0081114A3 (en) | 1984-05-09 |
EP0081114B1 EP0081114B1 (en) | 1986-02-19 |
Family
ID=23278317
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP82110650A Expired EP0081114B1 (en) | 1981-12-07 | 1982-11-18 | Method of firing a pulverized fuel-fired steam generator |
Country Status (7)
Country | Link |
---|---|
US (1) | US4411204A (en) |
EP (1) | EP0081114B1 (en) |
JP (2) | JPS58106319A (en) |
CA (1) | CA1188931A (en) |
DE (1) | DE3269238D1 (en) |
ES (1) | ES8400816A1 (en) |
IN (1) | IN159615B (en) |
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WO1998054513A1 (en) * | 1997-05-27 | 1998-12-03 | Aventis Research & Technologies Gmbh & Co Kg | METHOD FOR NOx-LOW COMBUSTION OF COAL IN DRY ASH STEAM GENERATORS |
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- 1982-11-18 EP EP82110650A patent/EP0081114B1/en not_active Expired
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Cited By (10)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
GR920100316A (en) * | 1991-07-26 | 1993-05-24 | Evt Energie & Verfahrenstech | Furnace firing pulverized coal. |
WO1998054513A1 (en) * | 1997-05-27 | 1998-12-03 | Aventis Research & Technologies Gmbh & Co Kg | METHOD FOR NOx-LOW COMBUSTION OF COAL IN DRY ASH STEAM GENERATORS |
WO2009108739A2 (en) * | 2008-02-27 | 2009-09-03 | Alstom Technology Ltd | Air-fired co2 capture ready circulating fluidized bed heat generation with a reactor subsystem |
WO2009108739A3 (en) * | 2008-02-27 | 2010-04-22 | Alstom Technology Ltd | Air-fired co2 capture ready circulating fluidized bed heat generation with a reactor subsystem |
AU2009219351B2 (en) * | 2008-02-27 | 2012-03-01 | General Electric Technology Gmbh | Air-fired CO2 capture ready circulating fluidized bed heat generation with a reactor subsystem |
US8196532B2 (en) | 2008-02-27 | 2012-06-12 | Andrus Jr Herbert E | Air-fired CO2 capture ready circulating fluidized bed heat generation with a reactor subsystem |
CN101960218B (en) * | 2008-02-27 | 2012-12-05 | 阿尔斯托姆科技有限公司 | Air-fired CO2 capture ready circulating fluidized bed heat generation with a reactor subsystem |
EP2267366A1 (en) * | 2008-03-06 | 2010-12-29 | IHI Corporation | Method of controlling combustion in oxygen combustion boiler and apparatus therefor |
AU2008352211B2 (en) * | 2008-03-06 | 2012-05-31 | Electric Power Development Co., Ltd. | Method and apparatus of controlling combustion in oxyfuel combustion boiler |
EP2267366A4 (en) * | 2008-03-06 | 2012-06-06 | Ihi Corp | Method of controlling combustion in oxygen combustion boiler and apparatus therefor |
Also Published As
Publication number | Publication date |
---|---|
EP0081114B1 (en) | 1986-02-19 |
ES517833A0 (en) | 1983-11-16 |
JPS6071842U (en) | 1985-05-21 |
CA1188931A (en) | 1985-06-18 |
JPS6218821Y2 (en) | 1987-05-14 |
DE3269238D1 (en) | 1986-03-27 |
US4411204A (en) | 1983-10-25 |
ES8400816A1 (en) | 1983-11-16 |
EP0081114A3 (en) | 1984-05-09 |
IN159615B (en) | 1987-05-30 |
JPS58106319A (en) | 1983-06-24 |
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